Visākha visits Dhammadinnā and asks her a series of questions on personality, the Noble Eightfold Path, plastic forces (sankhārā), etc. Dhammadinnā explains these things to Visākha, who then consults the Buddha as to whether the explanations are correct.
The Buddha tells him that he himself would have given the same answers and praises Dhammadinnā's erudition (M.i.299f).
The sutta forms a kind of commentary on certain psychological terms, such as sakkāya, sankhāra, etc.
See Vedalla.