A good video series from the Dominicans on the thoughts of St. Thomas is available (free). They produce fairly short, approachable explanations of the Summa Theologiae. The series is called Aquinas 101 (a playlist is also available).

Today's lesson is #82 (on the Summa part III, Q 83 a 1) by Fr. Thomas White on the Sacrifice of the Mass:

https://www.youtube.com/watch?v=uprwTPCh0Eo

This particular video may be especially helpful to non-Catholics.