Review of all 4 parts:
This story at the start of the 20th anniversary year of the show refers back to the 10th anniversary story The Three Doctors which featured Timelord hero Omega having gone mad after being trapped in a world of antimatter for many hundreds of years. Just like that story we have Omega trying to return from the world of antimatter and using The Doctor to help him.
I would say in this adventure Omega's plot makes less sense and the logic of various things is unclear. The coincidence of Tegan's involvement is too much of a contrivance, the location of Amsterdam seems to have no reason, the way Omega goes about his plan and how others respond seems odd at times and the Ergon creature is unsatisfactory in concept and appearance. However, there is also a lot to enjoy in this serial.
There is plenty of drama and entertainment. The locations are nice. Peter Davison as The Doctor is on great form. Nyssa has some very good moments and is generally strong support. Tegan and the backpackers are OK. The guest cast of Timelords is great with Michael Gough as classy as always, Leonard Sachs and Elspet Gray effective in their roles and Colin Baker, before being cast as the 6th Doctor is excellent as Maxil. He has the same supercilious demeanour that he portrayed in his first season as The Doctor but while I did not enjoy that for The Doctor it is good for this role and he acts perfectly adding quality to every scene.
The use of Gallifrey in a story in the 20th anniversary season is appropriate and there are some nice touches to the political shenanigans. Omega himself is pretty bonkers and looks rather strange but there are some exciting moments and menace created by threats which work well.
The special edition version has some lovely effects. A makeover is needed for the Ergon and a few other moments but overall this is fun and nice to look at.
Therefore, despite the shortcomings of the plot I rate this as a pretty solid if unexceptional story overall.
My ratings: Part 1 - 7.5/10, Part 2 - 7/10, Part 3 - 8/10, Part 4 - 7/10. Overall - 7.38/10.