Special episode based on manga Navigation 39 entitled "The Secret Place". Its action takes place between episodes 5 and 6 of Aria the Origination.

Akari and President Aria take a walk in the eastern part of Neo-Venezia. The are spotted by Akatsuki, who follows them to try to greet Akari. They eventually end up at St Mark's Basilica, where Akari has a secret place, which is discovered by Akatsuki, Aika, and Alice.

[Source: Wikipedia]