Preparations have begun for Rose's big day, but there are tensions on both sides of the family that threaten to derail proceedings.

Thomas returns to Downton but is suffering an inexplicable illness, and Baxter suspects there's more to it than he's letting on.

When things don't go as planned, Robert is left red-faced and furious; Mary's secret is nearly blown.

Branson must make a difficult decision about Sarah; Simon returns to Downton to discuss the painting with Cora.

Rose's father-in-law, Lord Sinderby, has rented Brancaster Castle in Northumberland and invited the Crawleys to a shooting party.

Robert and Cora's relationship is strained; Blake's scheming starts to come to fruition; Mary is unexpectedly pitted against her love rival.

Despite Calleigh's protests, Delko gets involved with his dangerous father. Meanwhile, Horatio tries to save Yelina.

Marilyn tells Jack the truth about his father's involvement in the day's conspiracy. Jack uses her in an attempt to get to Phillip.

Marilyn gives Jack a lead that will help him find Gredenko, but they hit a wall as Phillip takes steps to cover his tracks.

Jack learns a devastating piece of news about Audrey upon his return to CTU.

Jack meets with two brothers who are instrumental in leading him to Gredenko's location.

Wayne risks his unstable health in order to fight Daniels for the presidency. Lennox and Karen call an uneasy truce in the face of the impending war. Doyle coerces Nadia into helping him dig up dirt on Milo, while Jack uses Gredenko in a sting operation that will lead them to Fayed.

A group of pirates takes over a luxury yacht, which threatens Delko's career; Calleigh and Delko reach the next level of their relationship.

The mystery ensnares marine biologist Laura Daughtery in California, and family man Richard Connelly fishing in the Gulf of Mexico. Meanwhile, in North Carolina, teenager Miles Bennett finds one of the creature's eggs.

Miles tries to form a bond with the new species; Laura and Rich try to get onto Sullivan's Island to learn about the mysterious carcass that washed ashore.

Lee and Cirko begin to theorize that the unidentified species is causing global warming by digging beneath the ocean bed.

Animal control officers question good friends Miles and Phil, the keepers of Nimrod, when a neighbour's dog is reported missing.

When Daughtery and Jackson encounter one of the unidentified species at sea, Daughtery fires a GPS tracking device into it in an effort to track it.

Cirko discerns the origin of the creatures while studying a sea orchid; when Laura returns home, she finds that her house has been ransacked by the government.