Action at Lanark
Background
After Berwick and Dunbar, minor uprisings against the English became more common in Scotland. What makes Lanark unique is that it's the first time William Wallace has taken part in fighting against the English.
A fracas broke out at a court that William de Heselrig, the English-appointed Sheriff of Clydesdale, was holding in Lanark. William Wallace was present and escaped with the help of an individual who may have been his wife.
According to the 15th century poem "The Wallace," written by Blind Harry, William de Heselrig killed William Wallace's beloved wife.
Battle
On May 3rd, 1297, William Wallace came by night with a small force and surprised William de Heselrig and slew Heselrig, reportedly out of revenge for the killing of his wife. Blind Harry's poem also states that Wallace dismembered Heselrig's body.
Sir Thomas de Grey of Heaton Castle, who was at the time in the company of the sheriff, was also attacked and left for dead in the melee of battle when the English were trying to defend themselves. Thomas de Grey was severely wounded and lay all night in the snow between two burning houses that the Scots had set on fire, whereof the heat from the fire kept life in him. He was found the next morning and carried off by William de Lundy and eventually recovered. The English had a garrison of 240 soldiers at Lanark, who were either slain or scattered once the attack started.
Aftermath
The Action at Lanark set the tone for William Wallace's leadership during the First War of Scottish Independence. It showed others in Scotland that resistance was possible and inspired rebellion across southern Scotland.