The main collection is in the 16th-century Benedictine Abbey building that links the two castle towers. Exhibits include a large collection of manuscripts, paintings, sculpture, vestments and tapestries. Most of the exhibits are religious in nature, such as the 12th-century Płock bible, but there are a few secular items, including the Charter of Płock from 1237 and some medieval weaponry. There's an annex in the brick building near the cathedral entrance.
A few select pieces steal the show – look for the delicately gilded 12th-century ciborium from Czerwińsk, and a first edition of Adam Mickiewicz's Pan Tadeusz.