Leela falls from the War Master's TARDIS and into the Time Vortex. She remembers watching her mother die when she was a girl and then wakes up, finding herself on the ground with a young man stood over her. He tells her that she tripped over a Trell, an anemone-like creature, and that he is Sholan of the Teus. He helps her up to take her to see Drah, whom he needs to see anyway to ask about why the Trell stopped oscillating for a time, and says that Leela is his mother.
Drah is concerned about Leela's apparent amnesia and sends Sholan away to the Citadel to continue his studies with Kraumer so that she and Leela can talk alone. Leela and Drah walk among the Trell, which are drawn to Leela, and discuss Leela's past. Whilst she remembers being on Gallifrey, Drah has known her to have lived on Nateus since she was born. The planet is a quantum anomaly thanks to the presence of the Trell, which can alter the usual rules of space and time and only speak to one or two people in a generation; when Drah tells her to touch one, she feels connected to the Trell and sees the Vortex.
Sholan apologises to Madam Renucha for his lateness and sits with Kraumer, the heir to the throne. During the lesson about plant life, Sholan tells Kraumer that the Trells had stopped oscillating for a time, meaning that Nateus was briefly visible, but Sholan is unconcerned. Renucha collapses, unable to breathe, and seems to be dead for a moment before she recovers, unaware of where she is.
Drah says that Leela could be a visitor of consciousness, come to Nateus and merged with the mind of Leela of the Teus. Visitors are rare but usually inhabit the bodies of dying leaders and help the people until the leader's death. A time shift occurs and Leela finds herself with a baby, Sholan, and tells him about how she once fell on a branch which went straight through her arm and had to be removed by her father. However, her scar has disappeared.
Leela shifts back to Drah, who sees her claim to have experienced a time shift as evidence that she is a visitor, being shown the life of her host by the Trell. The Trell stop, disconnected from the Vortex, and Drah hurries to the Citadel with Leela to let them know.
Madam Renucha tries to continue with the lesson but cannot remember what she was teaching and does not seem to know much about them. She suggests that they go to examine the Trell in person and has Kraumer lead the way.
Leela and Drah use a vehicle far in advance of the technology used elsewhere, having been built from knowledge given by a visitor. Drah does not wish to speak of the Time War, which is currently far away from Nateus but getting closer, and says that they need to remain hidden and keep the people ignorant of the war. Leela believes that this is pointless as the Daleks will find them and destroy them, but she suddenly shifts away again.
Leela sees Sholan playing a fighting game with Kraumer and, when Sholan loses, she tells him that he must practise more in order to become invincible for the coming war. He says that there will never be war on Nateus, but agrees to fight harder. Leela then shifts ahead fifteen years into the war, when the Trell are weakening and Nateus is led by Lord Warrior, whom she assumes is Kraumer.
Renucha "tests" Sholan's knowledge by asking him about what the Trell can do and how somebody might be able to take them away, something which would require the planet itself to be moved. She then says that she is going to teach the students how to make a communications device.
Leela visits the gardener of the Trell, who tells her that Sholan is long dead and that she destroyed him. She shifts again and finds herself stalking a Mirren with a younger Sholan and telling him that he needs to harden his heart and become a hunter. Although he wants only to capture the Mirren and keep it at the Citadel, she forces him to shoot it between the eyes.
Renucha completes her communications device able to reach beyond the Vortex, ostensibly to contact allies. She is about to test it when Kraumer orders her to switch it off and destroy it, which she promises to do outside. Once Sholan and Kraumer are gone, Renucha uses the device to contact a Battle TARDIS, identifying herself as Lady Zeno and sending a message for the War Council letting them know that she knows a way to obtain the protection of the Trell. Before she can say anymore, she experiences a time shift into the future where the relocation of the Trell is about to commence and tells Drah that she is a Time Lord. Lord Warrior then kills Drah.
Leela finds herself with the dying Drah, who tells her that Renucha is actually a Time Lord and that she has to stop her and Lord Warrior. After telling her to go to the gardener, she dies. Leela then shifts back in time and finds herself running with Sholan and Kraumer from a Rudnig that she shot with an arrow. Leela and Kraumer cross a rope bridge whilst Sholan fends off the Rudnig with a branch, stunning it. It claws at the rope and Sholan and Kraumer fall, Sholan hitting his face against the wall and getting the scar that Lord Warrior has. Whilst Leela is able to get Sholan to safety, Kraumer falls. Sholan is angry at Leela for always having tried to make him somebody that he was not and says that she has finally succeeded. As he is now next in line to the throne, he will now be known as Lord Warrior.
Leela shifts ahead and finds herself with the gardener, watching Nateus getting attacked. She learns that the gardener is Kraumer, having survived the fall, and that Lord Warrior and Renucha must be stopped from betraying Nateus. The Trell speak to Leela and tell her to "let him go".
Zeno tells Lord Warrior that she has lived the attack on Nateus many times but has never been able to see how it ends, always shifting back into the past. They go to Drah's lodge to use a transmat to get off-world. Leela and the gardener find them and an earthquake hits as Lord Warrior and Zeno disappear. Leela says that she has to go back in time to let Sholan make his own choice and asks the Trell to send her back to the bridge. When the bridge breaks, Leela allows Sholan to sacrifice himself to save Kraumer, telling him that his heart is his strength and that he must listen to it. She wails as her son hits the water below.
Leela finds that the future is much the same, but Drah is alive and Nateus remains hidden. Zeno has gone mad and is attacking the Trell, having been told to destroy them as the Time Lords cannot move the planet. Leela throws herself into the open Earth with Zeno. The Trell tell Leela that she has the choice between eternal peace in death or to risk forgetting about Sholan and to return to life. She chooses to live.
- Leela - Louise Jameson
- Sholan - Sam Hallion
- Kraumer - Will Kirk
- Drah - Sarah Douglas
- Renucha - Maxine Evans
- Warrior - Wilf Scolding
- Leela remembers the death of her mother and how her father once removed a branch which impaled her arm.
- Drah says that Leela is the widow of Reem.
- Nateus is surrounded by dark energy.
- Sholan kills a Mirren.
- Zeno becomes Custodian of the Trell.
- Leela shoots a Rudnig.
- Mother Tongue was recorded at the Moat Studios.
- This story is the first Gallifrey audio not to feature Romana, and the second - after Intervention Earth - to not feature Romana II. It is also the first to not feature Narvin.
- Maxine Evans in fact plays Lady Zeno for most of the episode, who has transferred her consciousness into Renucha as a "Visitor" to Nateus.