Torchwood are chasing aliens down the M4, when Jack accidentally blows a hole in the Severn Bridge, and the SUV hits the water. Whilst submerged Jack, Gwen and Ianto hear a strange noise, which, back at the Hub they realise is a cry for help. They track the cry to its source which turns out to be the deepest part of the Ocean - the Mariana Trench. Ianto rings old Torchwood flame, Carlie Roberts, who's an expert in marine geology, and Jack pulls strings with the US government to get them all on board the USS Calvin, an Arleigh Burke class destroyer, which is heading for the Trench. From there they board the Octopus Rock, the only submarine built to withstand the pressure at that depth, and follow the signal. But when the Submarine crashes, the team are left at the mercy of a hungry alien.
to be added
- Captain Jack Harkness - John Barrowman
- Gwen Cooper - Eve Myles
- Ianto Jones - Gareth David-Lloyd
- Carlie Roberts - Erin Bennett
- Sam Doyle - Angelo Tiffe
- Captain Cudlow - John Francis Harries
- Henry Goddard - John Lee
- Facebook and Twitter are mentioned.
- Jack claims to have mentioned the Doctor to UNIT.
- Carlie mentions expeditions down to the Mariana Trench in the 1950s and 1960s.
- Jack compares the submarine to the Red October.
- Carlie refers to the bottom of the Mariana Trench as Davy Jones' Locker.
- Sam refers to Gwen as being as sweet as a cupcake.
- The submarine is named the Octopus Rock after a song by the Sex Pistols, an English punk band.
to be added
- Ianto mentions Torchwood One and the Cybermen, referring to the Battle of Canary Wharf in 2007 in which Carlie was also involved. (TV: Army of Ghosts / Doomsday)
- Carlie talks to Ianto about his relationships with Jack and Lisa Hallett. (TV: Cyberwoman, TV: End of Days et al)
- The Fourth Doctor, Romana I and K9 would later visit the Mariana Trench in 2040. (AUDIO: Phantoms of the Deep)
- Sam warns that Jack's son will likely fall one day, like his own died on his burning planet. This foreshadows the death of his grandson in TV: Children of Earth: Day Five.