Write SQL query to solve given problem: What is the average number of stars assigned to The simpson 20s: S20-E12? What is the said episode all about?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT AVG(T2.stars), T1.summary FROM Episode AS T1 INNER JOIN Vote AS T2 ON T2.episode_id = T1.episode_id WHERE T1.episode_id = 'S20-E12';