Tidal Wave (audio story)

Tidal Wave was the second story in the audio anthology, Assembled, produced by Big Finish Productions. It was written by Guy Adams and featured Jemma Redgrave as Kate Stewart, Ingrid Oliver as Petronella Osgood and Katy Manning as Jo Jones.

Publisher's summary
When an experimental tidal power generator needs its eco-friendly credentials checked, Kate Stewart calls in an expert.

Soon, Jo Jones is bound for "Project Charybdis" in the South Atlantic, along with an awestruck Osgood.

But out at sea, a treacherous plan is set in motion to awake an ancient race. Beneath the seabed an army is sleeping - an army of Sea Devils!

Plot
Kate meets Jo, who is livestreaming herself whilst chained to a pharmaceutical testing facility to protest experimentation upon animals. The facility is closed down after UNIT redirects anybody visiting the websites of the companies that use it to Jo's livestream; she agrees to help Kate with whatever it is that she needs, although she has lost the key.

Kate asks Jo to inspect the trial run of Project Charybdis to determine whether or not the mobile tide turbine is ecologically friendly and introduces her to Osgood, who is starstruck and joins her on the mission. Osgood and Jo fly to Ascension Island and meet with Dr Sale, the impatient chief engineer of Charybdis, and Captain Burmaster, who shows them to their quarters and locks them inside. Burmaster shoots and kills Dr Sale and reports to Jastrok before using the turbines to send a pulse into the seabed.

Commander Krelix, a Sea Devil, awakens and revives Elite Groups Four and Five, believing that they are under attack. Elite Group Four breaches the Charybdis. Upon learning that contact has been lost with the Charybdis, Kate takes a plane to Ascension Island and searches for it by boat. Krelix detects it and sends Elite Group Five to intercept it.

Osgood's mobile phone has no signal despite the resilience of the UNIT network, indicating that a signal dampener is in operation, and Jo suggests that Burmaster might be hypnotised. Osgood sends out a basic SOS and Jo picks the lock with Osgood's penknife. They go to get weapons from the equipment room and find Dr Sale's body, after which they eject themselves from the Charybdis in escape suits. Upon putting them on, they hear over the radio that Burmaster has set the Charybdis to self-destruct.

The Charybdis is destroyed and Osgood and Jo are recovered by Kate. Elite Group Five climb aboard and attack, so Osgood sends Jo in a submersible towards the Sea Devil base, sending out a message for peace. Krelix considers destroying it but, upon seeing that only one person is aboard, he has her sedated and taken for interrogation. Jo tells him about Burmaster and how he is being controlled by the Silurians; Krelix, whose warriors have placed thermic mines beneath the vessel, realises that Jastrok has attacked the base to release Krelix's bioengineered weapons. This is confirmed when a control signal agitates them and the breeding pens are breached.

Persuaded by Jo, the Sea Devils return to hibernation to protect themselves and await a day when they and the humans can make peace. Krelix saves Jo by taking her to her submersible after she was knocked out by the tremors, leaving the stranded Burmaster behind. The creatures leave the caves and swim towards the United Kingdom and Kate and Osgood go to deal with them, inviting Jo to join Yates and Benton at the debriefing.

Cast

 * Kate Stewart - Jemma Redgrave
 * Osgood - Ingrid Oliver
 * Jo Jones - Katy Manning
 * Anthony Burmaster - Nicholas Asbury
 * Laura Sale - Alex Tregear
 * Krelix / Sea Devils - Nicholas Briggs

Worldbuilding

 * Kate speaks Spanish.
 * Project Charybdis is a seventy-metre-long mobile tide turbine.
 * Osgood suffers from seasickness and once threw up on a pedalo.
 * Osgood wears a velvet jacket.
 * Jo once trekked from Tarapoto to Lima by donkey, carrying rare frogs that a vendor was turning into smoothies.
 * Jo tells Osgood a story, the moral of which is not to wear a miniskirt on Velbos.
 * Jo jokingly asks about Travel Scrabble.
 * Osgood has a penknife which also functions as a corkscrew and a saw. She is working on a sonic attachment.
 * Osgood uses Reptilia sapiens as a politically correct term to refer to Silurians and Sea Devils.

Continuity

 * Jo has recently met the Doctor again, but this time, he was in a younger body. (TV: Death of the Doctor)
 * Osgood mentions Jo's adventure with Ogrons, (TV: Day of the Daleks) Omega (TV: The Three Doctors) and the Giant maggots, (TV: The Green Death) explicitly musing that three Doctors at once in UNIT HQ must have been a once-in-a-lifetime event (suggesting that this takes place before Osgood's own such experience with the War, Tenth and Eleventh Doctors in the Black Archive) (TV: The Day of the Doctor).
 * Osgood says that dying on a UNIT plane is more likely than a commercial one. Her first Zygon double would later be killed by on a UNIT plane. (TV: Death in Heaven)
 * Krelix invokes the name of Dagon. (PROSE: All-Consuming Fire)
 * Jo recalls that the Third Doctor's attempt to broker peace with the Sea Devils was ruined by and Walker. (TV: The Sea Devils)
 * Jo mentions sailing down the Yangze in a tea chest. (TV: Death of the Doctor)