So your question is mainly related to Section 120 of the Indian Penal Code(IPC)
A conspiracy is an agreement between two or more persons to commit an offence. The mere meeting of mind of the parties to commit an offence is a conspiracy and is sufficient to impose liability. However, in the case of conspiracy, to do a legal act by illegal means, there ought to be some overt act which is committed by parties to the agreement.
A plain reading of Section 120A gives ingredients of Criminal Conspiracy which are as follows;
- There should be two or more persons.
- There should be an agreement between themselves.
- The agreement must be to do or cause to be done - an illegal act or a legal act by unlawful means.
Sec 120B deals with the punishment of criminal conspiracy. This section is in two parts i.e. sec 120B (1) and 120B (2).
Section 120B of the IPC talks about Punishment of criminal conspiracy:
- Whoever is a party to a criminal conspiracy to commit an offence punishable with death, imprisonment for life or rigorous imprisonment for a term of two years or upwards, shall, where no express provision is made in this Code for the punishment of such a conspiracy, be punished in the same manner as if he had abetted such offence.
- Whoever is a party to a criminal conspiracy other than a criminal conspiracy to commit an offence punishable as aforesaid shall be punished with imprisonment of either descripttion for a term not exceeding six months, or with fine or with both.
Section 120B(1) deals with heinous and grievous offences. However, section 120B(2) deals with minor criminal offences. Any person, who is a party to a criminal conspiracy to commit an offence is punishable under section 12B.
Regarding your 2nd doubt about 120B(2) in the case of a robbery and murder -
An act done in pursuance of this agreement is part of the criminal conspiracy. Hence, it is immaterial that the act done is the ultimate object or was merely incidental to the object. Thereby if there is an agreement to commit an offence, and two or more persons agree to do that illegal act or cause to be done an illegal act, it amounts to criminal conspiracy even if the object of the agreement is not fully accomplished.
So if 3 people planned to do a robbery and 1 person out of them murders the person getting robbed, then all of them will be charged together for mother (But can vary depending on the fact that that there was an eyewitness you can prove that other people didn’t do the murder and were innocent (they will be held liable to the robbery part). This is something like Vicarious Liability.
I hope this solves your query.