The RMS Achilles is a Royal Naval submarine commanded by Sean Ramsey, the leader of the Immunes which comes into conflict with the USS Nathan James (DDG-151) throughout season 2.
After the Red Flu pandemic, Immune leader Sean Ramsey began using the Achilles as his floating base of operations. The submarine came into conflict multiple times with the USS Nathan James, but never ended with either side sinking the other. In Cry Havoc Ramsey has the other Immunes form a blockade to force the Nathan James into battle with his sub while having a shore team set up an anti-ship missile battery in case the Nathan James gets close to shore. During the battle that follows, the Nathan James exhausts all of its weapons but four shells for its 5/62-inches Mark 45 Naval Gun which is a surface weapon. In response, Chandler decides to turn the battle into a surface fight and forces the Achilles to the surface by drawing it near shore where its too shallow for the sub to stay underwater. This brings the Nathan James into range of the Achilles shore battery, but a team led by Mike Slattery takes out the shore team. Once the Achilles is on the surface, the Nathan James starts turning to face the sub to use its cannon, but the sub is faster and prepares to fire torpedoes before the Nathan James can complete its turn. However, as the sub is about to fire, Slattery gains control of the missile battery and fires it on the Achilles, damaging it and stopping the attack. While the crew wants to abandon ship, Ramsey refuses and demands they keep fighting at gunpoint. Having finished its turn, the Nathan James opens fire on the defenseless Achilles with its cannon while Slattery fires the remaining missiles from the battery at it. The combined fire sinks the Achilles though Ramsey survives and is left trapped on the bottom of the ocean in his sunken ship with a dead crew.
The Achilles is equipped with the typical weapons of a submarine including torpedoes. It also possesses missiles which can be converted into a shore battery.
The ship was commanded by Sean Ramsey during the viral outbreak and repeatedly engaged the Nathan James which eventually sank it.