Arc of Infinity is a story from the 20th Season of Doctor Who, and sadly gets mired down in that anniversary. The story stars Peter Davison as the Fifth Doctor, alongside Tegan and Nyssa. The plot was rather slow, and pacing off a bit.
The highlight is the guest appearance of future Sixth Doctor Colin Baker as Commander Maxil.
If you are trying to complete a collection of Doctor Who DVDs, this is clearly a must buy. If you are a solid fan of the show, also a buy as this is better than a lot of Doctor Who produced at this time. If you are trying to see what Doctor Who is about, there are better stories out there to start with, such as "Spearhead From Space," "City of Death," and "The Caves of Androzani."