With the TARDIS immobilised, Ace realises that their own lives are as much in danger as those of the fleeing inhabitants. The race is on to escape the destruction of Tarsus Six and the devastating shockwave that will follow, reaching out and destroying everything in its wake.
The TARDIS lands on a space station in the 49th century. The station orbits a world called Tarsus Six, part of a system that has been heavily colonised by humanity. It's a world in chaos, and for good reason: it only has minutes to live. The planet's sun, Tarsus Ultra, is a red giant, but something is wrong: it's tearing itself apart, not as a supernova, but in some other, unknown way. Truthfully, the damage is done; the star has already released a massive shockwave that has destroyed the inner colonies, and is closing in on Tarsus Six. Ace wants to save everyone, but the Doctor sadly tells her they can't—it's too late.
Ships are fleeing the planet, some making it, some not. It's the same on the station. The Doctor and Ace meet a ship captain, OhOne, and his first officer, KaySix. Masquerading as authorities from Earth, they get themselves onto the Obscura, the last ship leaving the station. They have the TARDIS brought aboard as well. On the way, Ace sees a group of cultists who worship the star and believe the shockwave will cause them to evolve. She makes contact with one girl in the group, NineJay, and at the last minute uses the ship's transmat cubicle to bring her aboard and save her life—though it angers some of the passengers, who blame the cultists for the destruction of the star.
The ship, with the Doctor's help, barely outruns the shockwave just as it destroys the station and Tarsus Six. The Doctor takes Ace to check on the TARDIS, and tells her that they are trapped for now, as the TARDIS can't leave due to interference from the shockwave. She asks why they are here; he tells her they are here to rob a bank vault.
Ace is understandably disturbed, but the Doctor promises that they are not actually stealing; the artefact he wants belongs to him, sort of. It's a cube called the Voice of Stone, in the locals' vernacular. They had to come here, because the Voice's location can only be verified here and now. The Doctor says it will speak to him, if he can get to it. They reach the vault, and Ace goes to distract a sound-seeking security drone that has been following them, thus buying them some time. The Doctor has found the vault to be already unlocked—and the Voice has been stolen. Suddenly the alarms go off, and they are arrested by the captain.
It is revealed that they are not diplomatic envoys. The Doctor reveals that the Voice is missing, and that they did not steal anything. They are interrupted when the engines suddenly fail. OhOne takes them with him to check the engines, as the door won't lock without power; the Doctor restores the power, but the engines are still dead. The Doctor bargains for their freedom, in exchange for helping with the situation. KaySix discovers that the engines were actively sabotaged; security reveals that it was the girl, NineJay, whom Ace rescued. Ace goes to find her, and finds her in Ace's stateroom. NineJay reveals she was sent to join the ship so she could disable it, thus allowing everyone aboard to die in the shockwave and evolve; she says she almost missed the ship, and is grateful that Ace helped her. Ace lectures her for taking the choice from everyone aboard, but NineJay is unpersuaded; and she reveals that she took the Voice of Stone, or rather, "set it free". And now the shockwave is beginning to arrive; its outer fringes act as a warning shot against the ship. Time is running out.
NineJay gives the Voice to Ace, and reveals that she used the ventilation shafts to sneak around. The cube feels and seems to be alive; it reminds Ace of the TARDIS, and she thinks it is from Gallifrey. It responds to her, stunning NineJay. KaySix arrives and tries to break into the stateroom; Ace escapes through the vents, with NineJay following.
As they make their way to the engine room, OhOne broadcasts an announcement, indicating that the ship cannot escape the shockwave. However, they have a plan to use the main power unit to create a counterforce that will shield them until the shockwave passes. The ship will then be adrift, but can be recovered later with its passengers. However, someone must stay with the power unit to trigger it, and will die in the process. OhOne volunteers.
Ace arrives after the power unit is launched. Ace tells the Doctor her story, and turns over the Voice to him. The wakes up. It releases an image of a strange man, who is revealed to be the Doctor, which speaks to both of them. It can't hear them, but it remembers the conversation, and speaks accordingly. He acknowledges that he sent messages to the previous six, and asks the Seventh Doctor for help. The Seventh Doctor and Ace must save OhOne... who is on his way to die right now. He also tells the Seventh Doctor to be nicer to Ace. Ace realises that this means the Doctor will survive... but what about anyone else?
The shockwave is arriving, just two minutes away. OhOne has activated the counterforce wall, and is still alive... but the Doctor insists there is no way to save him now. He must be there to keep it active. Ace suggests that someone could take his place, but the Doctor won't let her do it. NineJay volunteers, however; she had long since chosen to die in the shockwave anyway. The Doctor is distrustful, thinking she will deactivate it and let everyone die; but she concedes that Ace was right to say that she can't take the choice from everyone else. It's a conflict of faith for her, not on her own behalf, but on that of everyone else. She transmats herself to the power unit, and transmats OhOne back.
With the shockwave passing by, the Doctor and Ace are free to go. Ace can't help wondering if there's some chance that NineJay was right—could she, somehow, have survived? They may never know. She cautions OhOne to be grateful—and maybe, he'll return the favour for someone in the future. They leave him shocked at the sight of the departing TARDIS.
- Ace claims that when you travel in the TARDIS, every day is different from the one before.
- Tarsus Six's sun, Tarsus Ultra, is a red giant.
- Ace has witnessed at least one supernova.
- The Tarsus system was seeded by humans in the 45th century.
- Captain OhOne is the captain of the Obscura while KaySix is the first officer.
- The Doctor and Ace pose as diplomatic envoys from the Earth Empire. Earth Central is the governing body of the Empire's colonies.
- Transmats are still in use in the 49th century.
- Ace refers to being captured as "an occupational hazard" while travelling with the Doctor.
- The Voice of Stone resembles a cube made of smoked glass. Ace believes that it was created on Gallifrey.
- The Doctor receives a message from his eleventh incarnation through the Voice of Stone, which is actually a Gallifreyan hypercube, telling him that he must save OhOne.
- The Eleventh Doctor describes his seventh incarnation as "probably one of [his] more circumspect periods."
- Ace recalls looking into the faces of Daleks (TV: Remembrance of the Daleks) and Cybermen. (TV: Silver Nemesis)
- The Eleventh Doctor refers to sending messages to his first six incarnations. (AUDIO: Hunters of Earth, Shadow of Death, Vengeance of the Stones, Babblesphere, Smoke and Mirrors, Trouble in Paradise)
- Time Lord hypercubes were previously seen in TV: The War Games and TV: The Doctor's Wife.