The OverdoseEpisode 4, The Brothers, can be watched via iplayer for a few more days.
When an abandoned baby is found in a car, Kurt Wallander oversees a large operation to find the missing father. Meanwhile, Linda is visiting a school all week to educate the pupils about the dangers of drugs. When a girl at the school has an overdose, the team begin to see connections between the two cases.
NB. The first two episodes, which were shown last year, Before the Frost and Mastermind are episodes 1 and 2 in the UK order but 1 and 6 in the Swedish order (according to Wikipedia) or 1 and 7 (according to Inspector-Wallander.org).
NB2. I'm informed that Rolf Lassgård who starred in an earlier series, which was based on the nine Wallander books, is the face of Wallander to most Swedes.