The Doctor and companions are captured by the Mechanoid, and taken to the Mechanoid city where they meet space pilot Steven Taylor, while Ian and Barbara decide to leave the Doctor.
This is easily the best episode of The Chase. The Mechanoids are quite impressive looking, and we are introduced to new companion Steven played by Peter Purves. He's very good here, and it's amazing it's the same actor who played Morton Dill in "Flight Through Eternity".
The final battle between the Daleks and Mechanoids starts out impressively, but quickly descends into the usual Richard Martin action scene silliness, with silly cartoony explosion effects overlaid on the action.
But the main reason this is the best episode of The Chase is the departure of Ian and Barbara. It's handled expertly the actors. William Hartnell, in particular, is excellent. The departure of Ian and Barbara is perhaps one of the best in the series.
Ian and Barbara were almost as much of a part of Doctor Who in its early years as the Doctor himself. William Russell and Jacqueline Hill's often under appreciated performances added immeasurably to the shows success.
The Chase, however, is one of my least favorite Doctor Who stories of all time. The opening two episode and closing two are half-way decent, but the middle two are absolutely horrendous. There's many other problems, from the writing to the direction to the terrible jazzy score from Dudley Simpson. I always feels a sense of relief after finishing watching The Chase.