The pair strike up a friendship almost at once. Mina learns that John has a supply of wellington boots that he may wish to use to trade with Whitecross and arranges for him to meet her the following day nearer the settlement.
The barge arrives early the next day whilst an excited Mina is preparing lunch for John. The barge is met by several members of the community. However, it is now manned by two young men called Kane and Grice.
The men are surprised by the reception committee, but bluff their way through, thinking no-one else has encountered the barge before. Pet is worried because she is sure she recognises Kane. The two men decide to take advantage of Whitecross hospitality and are entertained at the farmhouse by Charles.
Grice is less comfortable than Kane with the situation, especially when it becomes clear that former criminal Kane wants to case the joint.
At the barge there is almost a nasty accident, when John finds a gun and it goes off, almost hitting Greg and Lizzie. Greg wonders if the men are all that they say they are.
Greg and Charles question the men further but don't get anywhere. When Mina arrives on the scene still expecting John, Kane lies about the former owner of the barge. He tells her that he had to go back to his settlement to look after his sick wife. Mina knows this to be a lie as John had told her he was unmarried. She also notices that Kane is wearing John's hat and is now convinced that all is not well.
Greg and Lewis go to the barge to pick up the wellington boots and also find wood alcohol aboard. Mina tells Ruth about her concerns.
Pet remembers why she recognises Kane. He had been convicted of murder by a jury on which her father had served. Mina searches the length of the canal and to her horror finds John's dead body in the water with a knife lodged in his back.
Back at Whitecross she confronts Kane, demanding that he checks if the knife she has retrieved from John's body fits his empty sheath. Kane dismisses her allegations and leaves the farmhouse with Grice, after Mina tries to attack him with the knife.
The men return to the quarantine hut whilst Greg and the others discuss their next move. Later still Kane tells Grice he is going to get some insurance and leaves the hut. The next morning Lewis persuades Greg and Charles that he should mediate with the men before they take a more heavy-handed approach.
Lewis asks them to leave peacefully, but soon discovers that they have taken Lizzie and John hostage.
Kane demands horses with which to leave the settlement and shoots Lewis to prove he means business. Securing their requested horses they ride off with Lizzie and John, warning Charles and Greg not to follow them.
The pair ignore the threat and follow at a safe distance. Ruth and Pet can do nothing for Lewis as he is now dead.
Back on the barge again Kane and Grice have been drinking the wood alcohol not realising the danger in this. They both begin to feel groggy and ill.
John and Lizzie seize their chance to slip off the boat. Grice notices and fires his gun at them.
John and Lizzie reach safety unscathed. Due to the lighting of a match which ignites the wood alcohol fumes, the barge explodes spectacularly, knocking Greg, Charles and the children to the ground.
Mina teaches the children about 'parasites' and grieves the loss of John Millen.
Parasites is a gem of an episode thanks to wonderful guest appearances by Patrick Troughton and Kevin McNally. From its gentle introduction to its explosive climax the story is well paced and great to watch.
That we only have Patrick Troughton for the first few scenes is dissapointing, but this is more than made up for by the superb characterisation he brings to them. His interaction with equally gentle and misunderstood Mina is charming and ultimately ensures that his death at the hands of...