The interaction of interplanetary coronal mass ejections (ICMEs) with each other and with co-rotating interaction regions (CIR) changes their configuration, dynamics, magnetic field and plasma characteristics and can make space weather forecasting difficult. During the period of March 20–25, 2011, the Solar Terrestrial Relation Observatory (STEREO B) encountered a compound stream containing several interacting structures. Our analysis suggests that the stream consists of two ICMEs followed by an embedded ICME/CIR. The sudden appearance of ICME3 within the fast wind side of the CIR causes the temperature to drop suddenly to its lowest level in about 1.2 hours, by about factor of 36. The fast wind which follows the CIR influences not only ICME3's temperature but also its plasma β. In addition, third ICME impacts the CIR through expansion and deceleration. A Penetration of a forward pressure wave has appeared within ICME1. It is difficult to identify its resource, whether ICME2, the combined ICME3/CIR, or it is a remnant shock. Because of the compression, the temperature and plasma beta of second ICME and part of the first one have increased. Despite the presence of signatures from four large-scale interacting structures within the compound stream, it is difficult to reconcile the in-situ sequence with other remote sensing observations of CMEs and ejecta close to the Sun because of the large system of coronal expanding loops above the active region. Compound streams therefore remain difficult to interpret, and further understanding of the subject will depend on the future study of similar events.